1. Field of the Invention
The present invention generally relates to an image position detecting device and an image forming apparatus using the same, and particularly relates to an image position detecting device that accurately detects the position of images and an image forming apparatus using the same.
2. Description of the Related Art
There have been know image forming apparatus that sequentially transfer toner images with different colors formed by corresponding image forming units onto intermediate transfer member and then transfer the toner images onto recording paper at the same time. In such image forming apparatus, small velocity errors and velocity fluctuation of photosensitive member or the intermediate transfer member might cause misalignment between different color images (registration errors) that result in color shift. One solution for this problem is to provide an image forming apparatus with a photodetector or the like so as to detect positions of toner images and correct misalignment (see, for example, Japanese Registration Patent No. 3068865).
The following describes the general configuration of a related-art image position detecting device 10 with reference to FIG. 1. FIG. 1 illustrates the general configuration of the image position detecting device 10. The image position detecting device 10 shown in FIG. 1 comprises an image position detecting board 11, an LED (light emitting diode) 12 as a light emitter, which is mounted on the image position detecting board 11, and a light receiving unit 13. Light emitting sources, including the LED 12, may not be mounted on the image position detecting board 11. The light receiving unit 13 includes two light receivers (photodiodes D1 and D2) (FIG. 2) spaced apart by a predetermined distance in a moving direction of an intermediate transfer member 14 as an image carrier.
In the image position detecting device 10, the LED 12 mounted on the image position detecting board 11 irradiates light onto a predetermined position in a predetermined direction. The irradiated light is reflected by the moving intermediate transfer member 14 and a toner image 15 formed on the intermediate transfer member 14. The reflected light is received by the light receiving unit 13.
The following describes an exemplary circuit configuration of the image position detecting device 10 with reference to FIG. 2. FIG. 2 is an exemplary circuit diagram of the image position detecting device 10. More specifically, FIG. 2 is a circuit diagram of the light receiving side of the image position detecting device 10. In FIG. 2, the two light receivers (photodiodes D1 and D2) are shown, which receive the reflected light from the intermediate transfer member 14 and the toner image 15. The photodiode D1 converts the received light into a photoelectric current. Then, a resistor R1 performs I/V conversion to convert the photoelectric current into a voltage. The voltage is amplified by an operational amplifier Z1 so as to be output as a toner image detection signal SIG1 as a first analog signal.
The photodiode D2 is disposed closest to and downstream (rear side) of the photodiode D1 in a moving direction of the intermediate transfer member 14. The photodiode D2 converts the received light into a photoelectric current. Then, a resistor R2 performs I/V conversion to convert the photoelectric current into a voltage. The voltage is amplified by an operational amplifier Z2 so as to be output as a toner image detection signal SIG2 as a second analog signal.
The voltage of the toner image detection signal SIG1 is clamped by a diode D3 and resistors R3 and R4 so as not to fall below a predetermined minimum level. The two toner image detection signals SIG1 and SIG2 are input to a comparator Z3 as a comparison output unit, which compares the signals SIG1 and SIG2 to determine whether the level of the signal SIG1 is higher than the level of the signal SIG2 and outputs the result as a binary image detection output signal SIG3.
FIG. 3 is a timing chart of the toner image detection signals SIG1 and SIG2 and the image detection output signal SIG3. Referring to FIG. 3, as the lights received by the photodiodes D1 and D2 vary depending on the light intensity of the LED 12 and the color of the toner image transferred by the intermediate transfer member 14, the signal levels of the toner image detection signals SIG1 and SIG2 may vary as shown in (A)-(C) of FIG. 3, for example. However, the cross point of the toner image detection signals SIG1 and SIG2 is preferably maintained in a constant position (d) even under different conditions. This is because the toner position is determined by converting the time interval of the cross point timing into a distance.
The following is an example of calculating, with use of mathematical expressions, the cross point of toner image detection signals SIG1 and SIG2 obtained when an ideal toner image is detected by the ideal image position detecting device 10. When
                                                                        Dp                ×                V                ⁢                                                                  ⁢                1                ×                Vc                            +                              Dp                ×                V                ⁢                                                                  ⁢                2                ×                Vc                            +                              Dg                ×                V                ⁢                                                                  ⁢                1                ×                V                ⁢                                                                  ⁢                2                                                    V              ⁢                                                          ⁢              1              ×              V              ⁢                                                          ⁢              2                                ≦          Dt          ≦                                    2              ⁢              Dp                        +            Dg                          ,                            (        2        )            where Dp is the size of the individual photodiodes D1 and D2 in the moving direction of the intermediate transfer member 14, Dg is the distance between the photodiodes D1 and D2 in the moving direction of the intermediate transfer member 14, Dt is the size of the toner image in the moving direction of the intermediate transfer member 14, V1 is the peak level of the toner image detection signal SIG1, V2 is the peak level of the toner image detection signal SIG2, and Vc is a clamping voltage, then the moving distance X from a toner image detection starting point of the photodiode D1 to the cross point in the moving distance of the intermediate transfer member 14 is given by the following expression:
                    X        =                                                            V                ⁢                                                                  ⁢                1                ×                Dt                            +                              V                ⁢                                                                  ⁢                2                ×                Dg                                                                    V                ⁢                                                                  ⁢                1                            +                              V                ⁢                                                                  ⁢                2                                              +                      Dp            .                                              (        3        )            
When V1=V2, then Expression (3) can be replaced by the following expression:
                    X        =                                            Dt              +              Dg                        2                    +                      Dp            .                                              (        4        )            
That is, as long as Dt satisfies Expression (2), even if the peak levels V1 and V2 vary due to the condition of the image carrier (intermediate transfer member 14), such as vertical movement and torsion of the image carrier, the image position detecting device 10 can reduce the influence of the condition of the image carrier by adjusting amplification factors of the operational amplifiers Z1 and Z2 to satisfy V1=V2.
In the above-described image position detecting device 10, the operational amplifiers Z1 and Z2 have the same amplification factors so as to satisfy V1=V2. Accordingly, as shown in FIG. 4, when a toner image having a size greater than a predetermined size (Dt>2Dp+Dg) is detected, the peaks of the toner image detection signals SIG1 and SIG2 overlap each other (SIG1(max)=SIG2(max) shown in FIG. 2).
Therefore, the comparator Z3 cannot determine whether the level of the toner image detection signal SIG1 is higher than the level of the toner image detection signal SIG2 while the signals SIG1 and SIG2 remain at the same level, so that the image detection output signal SIG3 remains undetermined during that period. Thus, the image position detecting device 10 cannot perform accurate image position detection, color shift detection, or color shift correction. This problem cannot be solved by the method disclosed in Patent 1 that prevents the output in a normal condition from being undetermined by clamping the voltage.